Editor's Note: This paper is a peer-reviewed and edited version of a presentation delivered at NASF SUR/FIN 2012 in Las Vegas, Nev., on June 13, 2012. A printable PDF version is available by clicking HERE.

Abstract
Copper-tin (Cu-Sn) layers are well known and frequently used to avoid the use of nickel in many decorative and some technical applications. Consequently the plating of bronze has been with us for a long time. At present industrial electroplating is still performed from solutions containing cyanide. There have been many attempts to substitute for cyanide, but so far only with minor success. The results have not been satisfying with regards to appearance, operating window, plating speed and other properties. Therefore the cyanide-free electroplating of bronze is still just a wish for many platers. However, the regulatory requirements regarding health and environment are increasing and the problem in getting a license for cyanide, especially in China, has become severe. Therefore the need for alternative cyanide-free electrolytes is high. This paper reviews some basic chemistry and some concepts as well as recent developments in the electroplating of Cu-Sn from cyanide-free electrolytes. The influence on properties such as alloy composition, appearance and operating window are highlighted.

Introduction
Copper-tin (Cu-Sn) layers are well known and frequently used to avoid the use of nickel in many decorative and some technical applications. At present industrial electroplating is still performed from solutions containing cyanide. There have been many attempts to substitute for cyanide, but so far only with minor success. The results have not been satisfying with regards to appearance, operating window, plating speed and other properties. Therefore the cyanide-free electroplating of bronze is still just a wish for many platers. However, the regulatory requirements regarding health and environment are increasing and the need for alternative cyanide-free electrolytes is high. This paper reviews some basic chemistry and some concepts as well as recent developments in the electroplating of Cu-Sn from cyanide-free electrolytes.

Properties of Cu-Sn Alloys
Copper-tin alloys, especially bronzes, are one of the first metallic materials used by mankind. The excellent properties of bronzes, such as:

Good resistance to outdoor exposure,

White and yellow (gold) color with a bright appearance,

Hard and polishable,

Diamagnetic properties for high frequency connectors, and

Good wear resistance

have led to the development of electroplating solutions. A further advantage is the fact, that Cu-Sn alloys do not cause allergies and can be substituted nickel in some applications.

Electroplating History
The electroplating of copper-tin alloys is more than 170 years old, with the first activities reported around 1842. The constituents used in the first-mentioned electrolytes were cyanide complexed copper and tin in form of stannate. These components are still used in modern cyanide-based electroplating solutions.1

Cyanide-based Electrolytes
In the literature two types of electrolytes are described for the deposition of Cu-Sn alloys, one with 10-20 wt% of tin with a gold-yellow color and another with 35-50 wt% of tin for coatings with a shiny white appearance (speculum, mirror-alloy).

The baths containing cyanides (and stannates) are state-of-the-art and offer the following properties:

Some compositions contain heavy metals, e.g., lead as part of the brightener system, hence the coatings also contain lead. Nonetheless, more recent compositions have been developed which come with a lead-free additive system.

The basic composition and operating conditions of the two typical CN-based electrolytes are given in Table 1. The alloy composition can be controlled by the varying the concentrations of copper, tin, KCN and/or KOH.

Cyanide is a strong complexing agent. For this reason it is often used in the electroplating industry. In the case of copper-tin alloys, the use of cyanide allows and controls the simultaneous deposition to form a compact alloy with excellent properties. In addition, the anodic degradation products are not immediately problematic for the function of the electrolyte.

The major drawback of cyanides is the strong and rapid-acting toxicity. For this reason cyanides are more and more restricted, although safe handling procedures exist and their detoxification in wastewater can be controlled quite readily and effectively.

The restriction on cyanides applies to the import, storage, transportation and handling of dangerous goods. For all these activities a license is required and special requirements must be followed. This situation has become worse in recent years. In China, it is very difficult and time consuming to obtain a cyanide license. In addition, the upper limits apply to the total amount of cyanides in a given company in its entirety. This is clearly a driver for the continued development of cyanide-free electrolytes.

Cyanide-free Electrolytes
The electroplating of cyanide-free bronzes is not new. In his book on alloy electroplating, Brenner gave an overview on cyanide-free electroplating.1

The role of the cyanide is to stabilize (bind / chelate) the copper in the solution and allow the simultaneous, fine grained codeposition with tin. Copper forms the complex [Cu(CN)3]-2 with cyanide in solution. This complex exhibits a good solubility and is relatively stable. In electrolytes with soluble anodes, free cyanide ensures good anode corrosion.

All these functions should be provided by any potential substitute. Work has been done on several possible basic cyanide-free electrolytes. Some sample processes are listed below.

Increased copper codeposition in low current density area, leading to dark reddish deposits

Tarnishing of the deposits within a short period of time

Hazy, streaky, rough, non-compact deposits at makeup or after a short time thereafter

Limited maximum thickness with good appearance of the deposits

Narrow applicable current density range

Slow plating speed

Corrosivity of the electrolyte

Wastewater treatment issues

Electrolytes based on pyrophosphate, phosphonates and methanesulfonic acid have the least disadvantages and the highest potential to become accepted by the industry.

Other Considerations

Oxidation or breakdown of electrolyte components in the tartrate electrolyte
In the tartrate electrolyte, the tartrate ion decomposes at the anode. That leads to the formation of copper-containing precipitates.1

Oxidation of tin
In all electrolytes the oxidation of divalent tin [Sn(II)] to tetravalent tin [Sn(IV)] can take place at the anode, but the main reaction occurs with atmospheric oxygen.3 The resulting stannate(IV) is stable and accumulates in the electrolyte. In acid solutions, it forms hydroxides that precipitate by splitting off water and thus forming species like SnO2•xH2O and finally SnO2 sludge. This precipitate is often finely dispersed, or in a colloidal form and cannot be simply filtered off.

Alloy distribution and color uniformity
Non-uniform color is mainly caused by unfavorable alloy distribution as a function of the current density. If the alloy composition differs too much between high and low current density, then complex shaped parts, or parts plated in barrels will exhibit differing colors.

A pyrophosphate electrolyte8 deposits markedly more copper in the low current density area, as shown in Fig. 1. Therefore it is not the best choice for barrel plating, but applicable for rack plating.

In a basic phosphonate electrolyte, the change in copper concentration as a function of the current density is quite strong and at low current densities (Fig. 2), the deposits show a reddish / brownish color, while it is white at "normal" operating current densities. The addition of a selective chelating agent for copper (sulfur-containing additive) changes the situation completely (Fig. 3). With the addition of the additive to the phosphonate electrolyte, the alloy distribution is much more homogeneous and the electrolyte is now suitable for barrel plating without color uniformity problems.